I should note that I am in no way trying to start an argument of sorts or bash people with this topic. I just want to see what characters some of you here think has been twisted around by the fandom so much that their canon portrayals have pretty much become forgotten.
For me, Meiling, Kaguya, and Tenshi come to mind. The former is lazy and incompetent instead of a "just average" gatekeeper who gets her job done properly most of the time. The moon princess is... Do I even have to say how different she is from her enthusiastic and cheerful portrayal in canon? The latter is changed from "I like to fight" into "I like to get beaten up..."
Yukari is more of "misused" than she is changed a lot from her canon portrayal. Long answer short: she's reduced to nothing but a plot device character by the fandom. Most fan fictions, especially crossovers, I've come across has her being responsible for incidents or doing something out of boredom. I feel like crying every time I see a story summary that says something like "Yukari is bored, so she <insert an activity here>."
And there's Flandre and Yuuka being "ROFL stomp" characters most of the time. Sure, they are powerful, but some people make them more powerful than I think they should be. In fact, some people make the characters in the series more powerful than they should be. I once had to deal with a frightening fanboy who bombarded me nonstop for making the girls lose to characters from other series who are legitimately more powerful than them in one of my stories. He considers Touhou characters to be godly and unbeatable. The only other series he considers more powerful than Touhou is BlazBlue, but according to my friend, he exaggerates the power level of BlazBlu characters, making them look more powerful than they should be.
Anyone want to share their thoughts?